It should do good to heart and headWhen your soul is in my soul's stead; (55-56)
This is when writing and literature start to really matter. Not when you're having a fun night out with your buddies, but when you're really hurting, when you need healing in your heart and head. Terence gives us a little hint here that he's not feeling so great, that his own "soul" is in need of help, and that only poetry can "do good" for it.
And I will friend you, if I may, In the dark and cloudy day. (57-58)
Beer is good for a happy day, but on a sad and gloomy occasion, you'd better have some poetry at hand. Reading some sad poems can help us to prepare for the inevitable crud that life throws our way (or at least that's what Terence tells us).